A vehicle door system disclosed in Japanese Laid-Open Patent Publication No. 2006-240611 is known, for example. The vehicle door system in this publication has a first door panel and a second door panel, and the first door panel and the second door panel collaborate with each other and close a door opening in a vehicle body. The first door panel is relatively moved with respect to the second door panel and opens/closes a part of the door opening. Also, in this vehicle door system, the first door panel and the second door panel are relatively moved with respect to the vehicle body to open and close the whole of the door opening.
The first door panel slides in the horizontal direction along a pair of slide tracks provided on an upper part and a lower part of the second door panel. The second door panel is connected to the vehicle body through a hinge assembly, and the second door panel is rotatable around a vertical axis with respect to the vehicle body by using the hinge assembly as a fulcrum.